变温热源内可逆中冷回热布雷顿循环功率密度分析

用有限时间热力学方法分析内可逆变温热源中冷回热布雷顿循环,导出了无因次功率密度的解析式,由数值计算给出了燃气轮机功率密度特性,分析了循环中各热力参数对功率密度的影响,并对最大功率工况与最大功率密度工况下的主要参数进行了比较,得出了最大功率密度设计的优点和不足。     

闭式内可逆中冷回热布雷顿循环的功率优化

考虑高低温侧换热器、回热器和中冷器的热阻损失,以功率为优化目标,对恒温热源条件下内可逆闭式布雷顿循环的高低温侧换热器、回热器和中冷器的热导率以及中间压比的分配进行了优化。借助数值计算,分析了一些主要循环特征参数对最大功率及相应热导率和中间压比分配、双重最大功率的影响。     

Ecological performance analysis of an endoreversible modified Brayton cycle

An endoreversible closed modified simple Brayton cycle model with isothermal heat addition coupled to variable-temperature heat reservoirs is established using finite-time thermodynamics. Analytical expressions of dimensionless power output, thermal efficiency, dimensionless entropy generation rate and dimensionless ecological function are derived. Influences of cycle thermodynamic parameters on ecological performance and optimal compressor pressure ratio, optimal power output, optimal cycle thermal efficiency and optimal entropy generation rate corresponding to maximum ecological function are obtained and compared with those corresponding to maximum power output. The results show that cycle thermal efficiency improvement and entropy generation rate reduction are obtained at the expense of higher compressor pressure ratio and a little sacrifice of power output at maximum ecological function. The compromises between power output and entropy generation rate and between power output and cycle thermal efficiency, respectively, are achieved.     

Power and efficiency optimization for combined Brayton and inverse Brayton cycles

A thermodynamic model for open combined Brayton and inverse Brayton cycles is established considering the pressure drops of the working fluid along the flow processes and the size constraints of the real power plant using finite time thermodynamics in this paper. There are 11 flow resistances encountered by the gas stream for the combined Brayton and inverse Brayton cycles. Four of these, the friction through the blades and vanes of the compressors and the turbines, are related to the isentropic efficiencies. The remaining flow resistances are always present because of the changes in flow cross-section at the compressor inlet of the top cycle, combustion inlet and outlet, turbine outlet of the top cycle, turbine outlet of the bottom cycle, heat exchanger inlet, and compressor inlet of the bottom cycle. These resistances control the air flow rate and the net power output. The relative pressure drops associated with the flow through various cross-sectional areas are derived as functions of the compressor inlet relative pressure drop of the top cycle. The analytical formulae about the relations between power output, thermal conversion efficiency, and the compressor pressure ratio of the top cycle are derived with the 11 pressure drop losses in the intake, compression, combustion, expansion, and flow process in the piping, the heat transfer loss to the ambient, the irreversible compression and expansion losses in the compressors and the turbines, and the irreversible combustion loss in the combustion chamber. The performance of the model cycle is optimized by adjusting the compressor inlet pressure of the bottom cycle, the air mass flow rate and the distribution of pressure losses along the flow path. It is shown that the power output has a maximum with respect to the compressor inlet pressure of the bottom cycle, the air mass flow rate or any of the overall pressure drops, and the maximized power output has an additional maximum with respect to the compressor pressure ratio of the top cycle. When the optimization is performed with the constraints of a fixed fuel flow rate and the power plant size, the power output and efficiency can be maximized again by properly allocating the fixed overall flow area among the compressor inlet of the top cycle and the turbine outlet of the bottom cycle.     

Power optimization of an endoreversible closed intercooled regenerated Brayton cycle

In this paper, power is optimized for an endoreversible closed intercooled regenerated Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite-time thermodynamics (FTT) or entropy generation minimization (EGM). The effects of some design parameters, including the cycle heat reservoir temperature ratio and total heat exchanger inventory, on the maximum power and the corresponding efficiency are analyzed by numerical examples. The analysis shows that the cycle dimensionless power can be optimized by searching the optimum heat conductance distributions among the hot- and cold-side heat exchangers, the regenerator and the intercooler for fixed total heat exchanger inventory, and by searching the optimum intercooling pressure ratio. When the optimization is performed with respect to the total pressure ratio of the cycle, the maximum dimensionless power can be maximized again.     

中冷回热布雷顿热电联产装置的火用性能分析

建立了恒温热源内可逆中冷回热布雷顿热电联产装置模型,基于火用分析的观点,用有限时间热力学理论和方法研究了装置的性能,导出了无量纲火用输出率和火用效率的解析式。讨论了总压比给定和总压比变化两种情形,优化了中间压比和总压比,通过数值计算分析了回热度、中冷度和高温侧热源温度与环境温度之比等参数对装置一般性能和最优性能的影响,研究了火用输出率和火用效率之间的关系,其特性关系为扭叶型。最后发现分别存在最佳的用户侧温度使火用输出率和火用效率取得双重最大值。     

变温热源内可逆中冷回热布雷顿循环功率密度优化

以功率密度为目标,用有限时间热力学的方法,通过数值计算,对变温热源条件下的内可逆中冷回热布雷顿循环的高、低温侧换热器的热导率分配和中间压比、循环总压比和工质与热源间的热容率匹配进行优化。分别得到了最大功率密度、双重最大功率密度和三重最大功率密度,并分析了热力学参数对高低温侧换热器的热导率最优分配、最佳中间压比、最大功率密度和双重最大功率密度的影响。     

Maximum overall efficiency for a solar‐driven gas turbine power plant

A general model for an irreversible solar‐driven Brayton multi‐step heat engine is presented. The model incorporates an arbitrary number of turbines (N_t) and compressors (N_c) and the corresponding reheating and intercooling processes; thus, the solar‐driven Ericsson cycle is a particular case where N_t, N_c → ∞. For the solar collector, we assume linear heat losses, and for the Brayton multi‐step cycle, we consider irreversibilities arising from the non‐ideal behavior of turbines and compressors, pressure drops in the heat input and heat release, heat leakage through the plant to the surroundings, and non‐ideal couplings of the working fluid with the external heat reservoirs. We obtain the collector temperatures at which maximum overall efficiency η_max is reached as a function of the thermal plant pressure ratio, and a detailed comparison for several plant configurations is given. This maximum efficiency is obtained in two cases: when only internal irreversibilities are considered and when both internal and external irreversibilities (which corresponds to the fully irreversible realistic situation) are simultaneously taken into account. Differences between both situations are stressed in detail. In the fully irreversible realistic case, it is possible to perform an additional optimization with respect to the pressure ratio, . In particular, this double optimization leads to a valuable increase in efficiency (between 34% and 65%) for a plant with two turbines and two compressors compared to the simple solar‐driven one‐turbine one‐compressor Brayton engine. Copyright © 2012 John Wiley & Sons, Ltd.     

Power optimization of an endoreversible Brayton gas heat engine

The power output of a simple endoreversible Brayton gas heat engine is analyzed and optimized. The endoreversible engine is defined as a power cycle in which the two processes of heat transfer from and to the surrounding heat reservoirs are the only irreversible processes in the Brayton cycle. A mathematical expression is derived for the power output of the irreversible heat engine. The power optimization provides the basis for designing a real gas heat engine and for a performance comparison with existing Brayton power plants.     

A Parametric Study of an Irreversible Closed Intercooled Regenerative Brayton Cycle

Entropy generation minimization technique is used in the analysis of an irreversible closed intercooled regenerative Brayton cycle coupled to variable-temperature heat reservoirs. Mathematical models are developed for dimensionless power and efficiency for a multi-stage Brayton cycle. The dimensionless power and efficiency equations are used to analyze the effects of total pressure ratio, intercooling pressure ratio, thermal capacity rates of the working fluid and heat reservoirs, and the component (regenerator, intercooler, hot- and cold-side heat exchangers) effectiveness. Using detailed numerical examples, the optimal power and efficiency corresponding to variable component effectiveness, compressor and turbine efficiencies, intercooling pressure ratio, total pressure ratio, pressure recovery coefficients, heat reservoir inlet temperature ratio, and the cooling fluid in the intercooler and the cold-side heat reservoir inlet temperature ratio are analyzed.     

具有热阻和热漏的再热布雷顿循环性能分析

用有限时间热力学方法建立了一个工作在恒温热源TH、TL之间,存在热阻、热漏和再热的定常流空气标准闭式布雷顿循环模型。导出了其功率、效率的一般关系并对其进行优化,得到循环的基本优化关系;分析了在傅立叶导热定律下再热对循环最优性能的影响。     

闭式内可逆回热布雷顿热电冷联产装置(火用)分析

应用有限时间热力学理论和方法建立了闭式内可逆回热布雷顿热电冷联产装置模型,导出了装置无量纲(火用)输出率和效率的解析式。通过数值计算分析了回热器热导率对(火用)输出率和(火用)效率的影响,发现存在临界压比,同时优化了压比,研究了热电比、制冷和供热温度等设计参数对最优(火用)输出率和(火用)效率以及相应最佳压比的影响,发现最优(火用)输出率时的设计压比要大于最优(火用)效率时的设计压比,最优(火用)输出率和(火用)效率均随冷用户温度的升高而减小,分别存在最佳的热用户温度使(火用)输出率和(火用)效率取得最大值,热用户温度对装置最优(火用)性能的影响比冷用户温度更为明显。     

Power,power density and efficiency optimization of an endoreversible Braysson cycle

The performance optimization of an endoreversible Braysson cycle with heat resistance losses in the hot- and cold-side heat exchangers is performed by using finite-time thermodynamics. The relations between the power output and the working fluid temperature ratio, between the power density and the working fluid temperature ratio, as well as between the efficiency and the working fluid temperature ratio of the cycle coupled to constant-temperature heat reservoirs are derived. Moreover, the optimum heat conductance distributions corresponding to the optimum dimensionless power output, the optimum dimensionless power density and the optimum thermal efficiency of the cycle, and the optimum working fluid temperature ratios corresponding to the optimum dimensionless power output and the optimum dimensionless power density are provided. The effects of various design parameters on those optimum values are studied by detailed numerical examples.     

塔式太阳能-超临界CO2发电系统集成与优化

  [目的]  传统塔式太阳能热发电效率较低,采用超临界CO₂（sCO₂）布雷顿循环集成太阳能发电可有效提高系统效率。  [方法]  采用联立方程法建立塔式太阳能集热发电（CSP）和sCO₂布雷顿循环集成系统的非线性规划数学模型以辅助系统分析与优化。模型包含太阳能集热子系统、sCO₂布雷顿循环以及高精度CO₂状态方程的约束,无需调用外部CO₂物性数据,可实现对集成系统任意数量的设计变量的同步优化。将模型应用于塔式CSP与sCO₂简单回热布雷顿循环和再压缩布雷顿循环系统的案例研究,优化系统并分析设计变量对系统效率的影响。  [结果]  研究结果表明:集成再压缩循环系统最大热效率达29.4%,高于简单循环系统的24.9%。再压缩循环的最优透平入口温度为901 K、最优膨胀比约为3;简单循环的最优透平入口温度为826 K、最优膨胀比皆大于3.2。  [结论]  系统存在最优的透平入口温度,提高透平入口温度可提高系统效率,但过高的温度会导致系统效率下降;系统存在最优膨胀比,膨胀比对集成再压缩循环系统的热效率影响较小,但对集成简单循环系统效率的影响较大。     

Finite-time heat-transfer analysis and ecological optimization of an endoreversible and regenerative gas-turbine power-cycle

This paper deals with the application of finite-time heat-transfer theory to optimize ecologically the power output of an endoreversible and regenerative gas-turbine power-cycle for infinite thermal-capacitance rates to and from the reservoirs. The expressions for power, thermal efficiency, and exergetic efficiency corresponding to the maximum ecological function for the gas-turbine cycle are presented. The effects of regeneration and hot–cold temperature ratio on power, entropy-generation rate, thermal efficiency and exergetic efficiency, all at the maximum ecological function, are determined. It is shown that both the power output and entropy-generation rate are increased significantly by the use of regenerators, and increase monotonically with an increase with hot/cold temperature ratio. The results further indicate that the thermal efficiency and exergetic efficiency are decreased by the use of regenerators and rise with an increase in the temperature ratio. By the introduction of the ecological function, the improvements in exergetic efficiency and thermal efficiency are evident.     

太阳能驱动闭式简单燃气轮机循环热力学优化

研究太阳能通过换热器的闭式简单燃气轮机循环有限时间热力学性能，导出内可逆循环的最佳功率与效率间的关系，并得到最大功率输出及其相应的效率界限。用压气机和涡轮内效率表征循环内不可逆性，可得实际不可逆循环的最优性能。所得结果对闭式简单燃气轮机装置热力参数的选择有定指导意义。     

Brayton refrigeration cycle for gas turbine inlet air cooling

In this paper, a new approach to enhance the performance of gas turbines operating in hot climates is investigated. Cooling the intake air at the compressor bell mouth is achieved by an air Brayton refrigerator (reverse Joule Brayton cycle) driven by the gas turbine and uses air as the working fluid. Fraction of the air is extracted from the compressor at an intermediate pressure, cooled and then expands to obtain a cold air stream, which mixes with the ambient intake. Mass and energy balance analysis of the gas turbine and the coupled Brayton refrigerator are performed. Relationships are derived for a simple open gas turbine coupled to Brayton refrigeration cycle, the heat rejected from the cooling cycle can be utilized by an industrial process such as a desalination plant. The performance improvement in terms of power gain ratio (PGR) and thermal efficiency change (TEC) factor is calculated. The results show that for fixed pressure ratio and ambient conditions, power and efficiency improvements are functions of the extraction pressure ratio and the fraction of mass extracted from the air compressor. The performance improvement is calculated for ambient temperature of 45°C and 43.4% relative humidity. The results indicated that the intake temperature could be lowered below the ISO standard with power increase up to 19.58% and appreciable decrease in the thermal efficiency (5.76% of the site value). Additionally, the present approach improved both power gain and thermal efficiency factors if air is extracted at 2 bar which is unlike all other mechanical chilling methods. Copyright © 2007 John Wiley & Sons, Ltd.     

Thermodynamic optimisation of the integrated design of a small‐scale solar thermal Brayton cycle

The Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermal Brayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18 m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate. Copyright © 2011 John Wiley & Sons, Ltd.     

普适内可逆热机循环模型的(火用)经济性能优化

用有限时间热力学方法分析工作在恒温热源TH、TL之间内可逆普适热机循环模型的经济性能,导出循环利润率与工质温比、热效率与工质温比的关系式;以及利润率和效率的特性关系。所得结果包含了内可逆D iese、lO tto、A tk inson和B rayton循环的有限时间经济性能。     

Thermodynamic analyses and optimization of a recompression N2O Brayton power cycle

Thermodynamic analyses and simultaneous optimizations of cycle pressure ratio and flow split fraction to get maximum efficiency of N₂O recompression Brayton cycle have been performed to study the effects of various operating conditions and component performances. The energetic as well as exergetic performance comparison with its counterpart recompression CO₂ cycle is presented as well. Optimization shows that the optimum minimum cycle pressure is close to pseudo-critical pressure for supercritical cycle, whereas saturation pressure corresponding to minimum cycle temperature for condensation cycle. Results show that the maximum thermal efficiency increases with decrease in minimum cycle temperature and increase in both maximum cycle pressure and temperature. Influence of turbine performance on cycle efficiency is more compared to that of compressors, HTR (high temperature recuperator) and LTR (low temperature recuperator). Comparison shows that N₂O gives better thermal efficiency (maximum deviation of 1.2%) as well as second law efficiency compared to CO₂ for studied operating conditions. Component wise irreversibility distribution shows the similar trends for both working fluids. Present study reveals that N₂O is a potential option for the recompression power cycle.